The Old Ones are an ancient race of demons or gods. They once ruled the mortal world, but were later cast out and locked away from reality. Being primal spiritual entities, they cannot ever truly be destroyed. Not all are Outside, however, as some, like the one known as the Sleeper, are on Earth, but imprisoned and entombed.
Even today, they can be still active, feeding on psychic energy, primarily the fear and awareness of them present in humanity. For that reason, the Venatori seek to wipe out knowledge, and thus influence, of the Old Ones in the Oblivion War. The existence of the Old Ones is also one of the last things wizard apprentices learn about in a traditional apprenticeship.
In the seriesEdit
In Grave Peril, Harry Dresden and Bob discuss what The Nightmare could be. Bob suggests that (since it was able to torture Micky Malone after apparently crossing a threshold uninvited) it could be one of the Old Ones. Harry shoots this idea down saying that, if were one of the Old Ones, guardian spells across the world would be alerting the White Council and the Gatekeeper.
In Dead Beat, when told that Outsiders were called up to fight for the Red Court, Harry Dresden reflects on the implications, noting that Outsiders were foot soldiers of the Old Gods and that only mortal magic can call up Outsiders.
In Backup, Thomas Raith explains the Oblivion War, and how it seeks to lock away the old ones, to Bob. He can tell Bob without endangering the mission since the skull isn't mortal, and his memories can't bind anything to the mortal world.
In "Cold Case", Molly Carpenter and Carlos Ramirez encounter a cult dedicated to worshiping and releasing the Sleeper. Ramirez states that the Sleeper's power has been growing due to Lovecraft's writings spreading the idea to humanity. This has given the Sleeper enough power to influence the world, forming a number of cults. Members of these cults slowly go insane, lose their humanity and turn into something inhuman. This transformation goes far enough to give cult members tentacles and the power to psychically interrogate people.